One important feature of known drug delivery devices is to provide means allowing a user to identify the dose selected and/or administered. For this purpose, known devices comprise a dial or number sleeve having numbers, letters or signs on its outer surface which are at least partly visible through the housing, e.g. a window in the housing. In some known devices the dial or number sleeve rotates during dose setting and/or during dose dispense, wherein only one single number, letter or sign corresponding to the selected dose is visible through the window. An example of such a known drug delivery devices is described in WO 2008/145171 A1.
Further DE 20 2008 011 175 U1 and WO 2010/020311 A1 disclose a similar mechanism having a housing with a housing wall and a first window provided in the same. A dial is rotatably disposed in the housing, on which dosing values are disposed in a spiral shape and are partially visible through the first window. A second window is located in the housing and is disposed displaceably in the longitudinal axis of the housing, wherein a rotational motion of the dial is synchronized with a longitudinal displacement of the second window, in order to display the currently set injection dosage through the first window and the second window. A first transparent lens arrangement is located in the first window, said arrangement acting on its own to distort the display values visible there through. A second lens arrangement is located in the second window, said arrangement acting together with the first lens arrangement to increase the legibility of a set dosage value visible through the first lens arrangement and the second lens arrangement, and thereby counteracting false readings of the selected dosage setting.
Although the first transparent lens arrangement distorts the display values visible there through, the values may still be seen which might confuse the user. In addition, the arrangement with different lenses which have to match with each other to allow only one dose value to be visible is complicated.